feat(orchestrator): update a single egress secret in place (0081)

Add a single-secret counterpart to reprovision_from_secret's restore-all:
update ONE egress token for a running bottle without a relaunch, for
refreshing a short-lived host credential (e.g. the Codex access token)
whose launch-time snapshot has expired.

- registry_store.store_agent_secret: per-key upsert (delete+insert of the
  one row), the counterpart of store_agent_secrets' replace-all.
- OrchestratorCore.update_agent_secret: set the in-memory token AND upsert
  the re-encrypted row under the bottle's env_var_secret, leaving other
  tokens untouched.
- POST /bottles/<id>/secret (cli-only) + client.update_agent_secret.

Refs #510, #512

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/orchestrator/service.py

@@ -379,6 +379,30 @@ class OrchestratorCore:
         self._tokens[bottle_id] = decrypted
         return True
 
+    def update_agent_secret(
+        self, bottle_id: str, name: str, value: str, env_var_secret: str,
+    ) -> bool:
+        """Update ONE egress token for a known bottle in place — the
+        single-secret form of ``reprovision_from_secret``.
+
+        Sets the in-memory token AND upserts the single re-encrypted row under
+        *env_var_secret* (the same key the rest of the rows are encrypted with, so
+        the whole set stays decryptable by a later ``reprovision_from_secret``),
+        leaving every other token untouched. Returns False if the bottle is
+        unknown.
+
+        Unlike ``reprovision_from_secret`` (which restores the values captured at
+        launch), this pushes a *caller-supplied* value — used to refresh a
+        short-lived host credential (e.g. the Codex access token) into a
+        still-running bottle without a relaunch."""
+        from .store.secret_store import encrypt_value
+        if self.registry.get(bottle_id) is None:
+            return False
+        self._tokens.setdefault(bottle_id, {})[name] = value
+        self.registry.store_agent_secret(
+            bottle_id, name, encrypt_value(env_var_secret, value))
+        return True
+
     # --- consolidated gateway ----------------------------------------------
 
     def gateway_status(self) -> dict[str, object]: